Each email address can only be used to sign up for a Quibi account once. If you or someone else previously signed up using that email address, you will receive a message when trying to create a new account that the email address is already associated with another Quibi account.
To regain access to that previously created account, select the "Sign In with Existing Account" button, enter your email address, and select the "Forgot Password" option. This will send a password reset link to your email address.
Once you've reset your password, you can log into the app and start a new subscription.